Process for preparing alkyl hydroxyalkyl cellulose ethers and the product obtained thereby



PRUiIESS FOR PREPARING ALKYL HYDROXY- ALKYL CELLULOSE ETHERS AND THEPROD- UCT UBTAINED THEREBY Albert B. Savage, Midland, Mich., assignor toThe Dow Chemical Company, Midland, Mich, a corporation of Delaware NoDrawing. Filed Apr. 30, 1956, Ser. No. 581,384

8 Claims. (Cl. 260-231) This invention relates to an improved processfor preparing certain cellulose ethers and to the ethers so prepared.More particularly it relates to a process for preparing alkylhydroxyalkyl celluloses having higher gel points and better aqueoussolubility characteristics than has been heretofore possible.

Alkyl cellulose ethers and alkyl hydroxyalkyl cellulose ethers of thewater-soluble type have been known for a long time and have found wideacceptance in the pharmaceutical, foods, and other fields. Thefabrication of such ethers or their use has usually been from aqueoussolution. The principal drawbacks and disadvantages of the prior knownmaterials has been in their poor solubility characteristics andproperties in water, and particularly in hot water. Thus, in thepreparation of aqueous solutions it has always been necessary to firstwet the ether with hot water and to cause dissolution subsequently bychilling the suspension. Even by employing such precautions thesolutions were always hazy due to the presence of insoluble solids whichhad to be removed by centrifuging when the cellulose ether was to beused from aqueous solutions. When such solutions were heated to about 50to 60 C. they gelled, destroying their useful properties. Additionally,the salt tolerance of such cellulose ethers has been very poor and inthe presence of salts the temperature at which gelation occurs islowered proportionately to the salt concentration.

It has been a peculiar property of the water-soluble alkyl cellulosesand alkyl hydroxyalkyl celluloses that, when previously prepared watersolutions are heated there is the expected decrease in solutionviscosity until the gel temperature is approached, whereupon, also asexpected, the viscosity rises sharply, but that on cooling the hotsolution or gel from temperatures near or above the gel point theviscosity of the solution follows a curve different from that observedduring heating. Thus, the viscosity of solutions of such ethers differs,at any given temperature and concentration, depending on whether thesolution is being treated toward or cooled from the gel temperature. Forsome uses this viscosity hysteresis is objectionable. It would bedesirable to provide watersoluble alkyl hydroxyalkyl celluloses whosesolutions, on heating and cooling, show no sigfinicant hysteresis. Pastattempts to overcome some .of the above difiiculties have consisted offorming various mixed carboxymethyl ethers such as carboxymethyl methylcellulose. Those mixed carboxymethyl ethers have succeeded in raisingthe temperature at which gelation occurs and have reduced the hysteresisbut have not avoided the difliculty of dissolution or improvedsolubility.

It is accordingly the principal object of this invention to provide aprocess for preparing certain cellulose ethers which are easilydissolvable in cold water' and which are completely soluble in coldwater. a

It is another object toproyide a newjclass of cellulose ethers havingthe above properties.

It is a further object to provide a new class of cellulose rates Patent2,949,452 Patented Aug. 16, 1960 ethers having temperatures of gelationwhich are higher than those of the prior known compounds of similarchemical structure, and whose solutions, on heating and cooling, Whileagitated, show no significant hysteresis.

The above and related objects are accomplished by means of a processwhereby cellulose is contacted with certain amounts of a basic materialat a relatively low temperature followed by etherification with anetherifying agent consisting of a combination of an alkyl halide andalkylene oxide in certain proportions. The products made in accordancewith the new process may be dissolved in cold water without priorwetting with hot water, are more soluble in water, have highertemperatures of gelation, and exhibit greater tolerance to salts thanthe prior known materials, and their solutions follow essentially thesame viscosity-temperature curves when heated and cooled.

Although any form of cellulose may be employed in the process, it shouldbe apparent that the form used must be capable of substantially uniformpenetration and of swelling by the basic material at the concentrationsand temperatures employed. Cotton linters being readily available,economical, and easy to handle represent a preferred form of celluloseto be used in the process.

The basic material may be any strong base which is capable of reactionwith the cellulosic hydroxyls to form an alkali cellulose. Thus, certainorganic amines and alcoholates as well as inorganic bases may be used,although for economic reasons the alkali metal hydroxides, andparticularly sodium hydroxide, are to be preferred. Where the basicmaterial is normally a solid it is employed in a solution and with thepreferred hydroxides it is most convenient to employ an aqueoussolution. It is additionally preferred to employ monofunctional basicmaterials to avoid the possibility of cross-linking between cellulosicmolecules which would complicate the subsequent etherification steps.

In the preparation af alkali cellulose, it is preferred to employ ratiosof basic material to cellulose of from about 1.2 to 28 moles to 1 mole.When sodium hydroxide is used, this ratio will be from 0.3 to 0.7 partsby weight of sodium hydroxide to each part of cellulose. To achieve sucha ratio with the common dipping procedures for preparing alkalicelluloses it has been found that an alkali hydroxide concentration offrom 30 to 55 percent by weight should be used. The alkali cellulose isprepared by contacting the cellulose with the strong base until thealkali cellulose has the above defined molar ratio of base to cellulose.In prior methods of preparing alkali celluloses it was considerednecessary to employ the caustic or other base either in excess or at anelevated temperature to achieve substantially uniform penetration andswelling of base throughout the cellulose and consequently to providesatisfactory solubility of the subsequently etherified product. However,that alkali cellulose after subsequent etherification resulted in aproduct having the previously discussed disadvantages, such as pooraqueous solubility and a relatively low gelation temperature. Whensubstantially less than 1.2 moles of base are used for each mole ofcellulose the products after subsequent etherification have a greatamount of insoluble material causing hazy solutions and non-homogeneousarticles unless such solids are removed by centrifuging. It has now beenfound that when the cellulose is contacted by dipping, soaking, or othermeans with the base in the concentrations and weight ratios mentionedabove and at about room temperature the cellulose is "substantiallyuniformly penetrated and slightly swollen alkali cellulose is etherifiedby employment in certain proportions of an alkyl halide in combinationwith a lower alkylene oxide 9. product having superior solubility andgelation properties results. It is preferred to use methyl chloride andethyl chloridebecause of their avail- 4 lower temperature will be usedthan when the less reactive butylene oxide is used. Also with the morereactive reagents such as ethylene oxide it is desirable to employ arelatively low temperature at the start of the ability and because thehigher alkyl halides are less 5 reaction and to gradually raise thetemperature as the reactive and produce ethers having less aqueoussolureactive reagent is consumed. In this manner the violent bility. Thealkylene oxides which may be used are polymerization reactions ofethylene oxide are avoided. those having from 2 to 4 carbon atoms sincethe higher The optimum temperature to be employed to provide an oxidesreact very slowly. Propylene oxide is preferred. easily controllablereaction may be determined by simple It is preferred to employe thealkyl halide at a molar preliminary experiments. The pressures which areused equivalency to the base used. When substantially less will becontrolled by the reagents and by the temperaalkyl halide is employedcomplete etherification of the ture employed. Although gauge pressuresof from 0 alkali cellulose does not result and the product requires to200 pounds per square inch may be used it is preneutralization toprevent gelling and to improve the ferred to operate at 100 pounds persquare inch or less. solubility. When an amount in excess of the statedThe alkyl chloride and alkylene oxide may be introquantity is used thegel point of the product is lowered. duced into the reactionsimultaneously or sequentially. The alkylene oxide is preferablyemployed at oxide For practical reasons it is preferred to introducethose to cellulose ratios when the oxide is used with methyl etherifyingreagents simultaneously. The reagents are halides of 0.22 or less partof ethylene oxide per part of conveniently brought into contact with thealkali cellulose cellulose, 0.25 or less part of propylene oxide perpart by evacuating the reaction vessel containing the alkali ofcellulose and at least 0.2 part but not more than 0.5 cellulose andsubsequently relieving the vacuum with part of butylene oxide per partof cellulose. When ethyl the etherifying reagents. or higher halides areused the amount of oxide or the The process of this invention produces aproduct in ratio of oxide to cellulose will be proportionately ina yieldof 83 to 95 percent of the theoretical yield. The creased. When noalkylene oxide is used, the ether exproduct may be washed with hot waterto purify it and hibits high solids or a great amount of insolublemateyet will dissolve readily in cold water without the necesrial. Whenexcess alkylene oxide is employed the ether sity of first being wettedwith hot water. The ethers so is ditficult to wash with hot water topurify the product, produced have gelation temperatures which are atleast since the product gels and is lost in the subsequent iso- 10 C.higher than previously known alkyl hydroxyalkyl lation steps. Thus, themaximum amount of oxide to cellulose ethers of similar chemicalstructure and in many he used is less than the limiting amount thatcauses cases they do not gel even at 90 C. The aqueous solugelationduring washing. The amount of oxide emtions prepared from these ethersare clear and substanployed will depend to some extent on the viscosityof tially ireeof insoluble solids. These solutions upon heatthe etherdesired. For high viscosity ethers it is most ing and cooling do notexhibit any significant amount of satisfactory to use from 0116 to 0.22part ethylene oxide hysteresis as is shown :by the prior ethers ofsimilar per part of cellulose while for low viscosity ethers it ischemical structure. desirable to use from 0.1 to 0.15 part ethyleneoxide per The products of the process of this invention find part ofcellulose. The other oxides are used propo-rutility in many widespreadand diversified applications. tionately. Thus, they may be emulsit'yn'ngagents for polymerization The decrease in insoluble solids exhibited bythe 40 reactions as well as emulsion stabilizers and thickeners productsof this invention is greater than is caused by for the latexes resultingfrom such polymerizations. They an equivalent increase in methoxylcontent. It is beare granulating agents for other polymerizationreactions. lieved that the alkylene oxide reacts more readily than Theyare bulk laxatives. They may be used in or as coatthe alkyl chloride inthe less penetrated regions of the ings on foodstufis and in the hotcasting of films and alkali cellulose, or else it aids reaction in theseregions. 5 sheets of cellulose ethers. They are useful in wall paper Theetherification is carried out under pressure at a paste, paint removers,and drilling muds. temperature of from 20 C. to 140 C. Although very Theoperation of the process and the advantages of high temperatures may beused, no beneficial result is the products of this invention will bemore apparent from attained by using such temperatures and the reactionmay the following illustrative examples wherein all parts, perbe violentand uncontrollable. When the temperature is 5 ceutages, and ratios areby weight. too low, the reaction is unduly prolonged. Since the Fifteensecond cotton linters pulp was dipped in an reactivity of the reagentsvaries it should be apparent aqueous sodium hydroxide solution atvarious temperathat the reagents varies it should be apparent that thetures and at varying ratios of hydroxide to cellulose. Each temperatureemployed will depend on that reactivity. resulting alkali cellulose wasthen broken up, loaded into Thus, when ethylene oxide is employed aconsiderably a pressure reactor and the latter was evacuated. The

Table I Examnle 1 2 3 4 5 6 (for comparison) Cannot NaOHin aq.soln 48.248.4 4 ,4, NaOH/l part cellulose 0.51.. 0.48 0.49. Dipping Temp. (O.)28. 28 28 31 32, Oxide Ethylene.-. Propylene.-- None. Oxide/1 partcellulose loade O 0 1.00 Alkyl chloride Ethyl. Methyl Alkylchloride/cellulose loaded 060 .65. Alkylehloride/lpartcellulcseconsumed. 59 .60. Temps. of etherification (O.) 2 hrs. 42,

3hrs.5275.

Pressure (Maxldnal) 142/45. Viscosity (2% in HOH at 20 C.) S02. Gelpoint C.) 62. Alkoxyl (percent). 22.9. Total D.S 1.36. Yield(Percent).-. 93. Solids (Percent) 3.

1 2 hrs. at 42-48" 0., 3 hrs. at 52-75 O.

w, the. v

identified alkylene oxide and alkyl chloride were loaded into thereactor and the reaction continued at temperatures below 100 C. untildetermined amounts or alkyl chloride had reacted. The products werewashed with hot water, and dried in an air oven. The results are listedin Table I. The solids were determined by centrifuging a 1 percentaqueous solution and observing the volume of insoluble material.

Example 6 was included for comparative purposes and shows that theabsence of the alkylene oxide produces a product having both a low gelpoint and high insoluble solids. The remainder of the examples show thatthe products of this invention have low insoluble solids and/ or highgel points. It is well known the commercially available grades of methylhydroxypropyl cellulose have gel points of no higher than 70 C. Thus theproducts of this invention exhibit gelation at temperatures of at leastC. higher than the corresponding ethers prepared by known processes.

EXAMPLE 7 Standard aqueous solutions were prepared from the ethers ofthis invention and of ethers prepared by prior known processes. Thesolutions were heated and then cooled and during the heating and coolingwere evaluated by a continuously recording torsional viscosimeter withagitation known as a Brabender Amylograp The measurement are listed inTable II.

Table I I Relative Viscosity Sample Cone.

heating cooling Methyl Cellulose made by prior methods (for comparison)0. 8

Methyl Hydroxypropyl cellulose prepared according to Example Methylhydroxypropyl cellulose prepared by prior methods (for comparison) 0. 7

coocqaam CDMOSOHJLMIO o acoooo omoooom ooqomimccnomoroocmcooi I wfi ihFrom the above tables the amount of hysteresis in the methyl celluloseprepared by the prior methods can be seen to be significant whereas withthe ether of this invent-ion the hysteresis is negligible.

I claim: 7

1. The process of preparing alkyl hydroxyalkyl ethers of cellulose whichare readily soluble in water, have higher gelation temperatures than theprior known ethers having the same substituents, and thetemperature-viscosity curves of whose solutions are essentially freefrom hysteresis on heating and cooling, which consists essentially incausing cellulose to absorb from about 1.2 to about 2.8 mols of aqueousalkali metal hydroxide per mol. of: cellulose, at a temperature fromabout 15 to about 30 C., and subjecting the resulting alkali celluloseat a temperature from 20 to C. to the etherifying action of an alkylhalide having from 1 to 2 carbon atoms, an amount approximatelyequivalent to the amount of said absorbed alkali metal hydroxide, and tothe etherifying action Of an alkylene oxide having from 2 to 4 carbonatoms, until substantially all of the alkyl halide and from about 0.1 toabout 0.5 part by weight of alkylene oxide per part of cellulose hasreacted, and recovering the resulting mixed ether of cellulose.

2. The process claimed in claim 1 wherein said alkene oxide and saidalkyl chloride are brought into simultaneous contact with said alkalicellulose.

3. The proces claimed in claim 1 wherein said alkali metal hydroxide isaqueous sodium hydroxide.

4. The process claimed in claim 1 wherein said alkylene oxide ispropylene oxide at an oxide to cellulose ratio of less than 0.25 to 1.

5. The process claimed in claim-1 wherein said alkene oxide is ethyleneoxide at an oxide to cellulose ratio of less than 0.22 to 1.

6. The process claimed in claim 1 wherein said alkene oxide is butyleneoxide at an oxide to cellulose ratio of at least 0.2 to 1.

7. The proces claimed in claim 1 wherein said alkyl halide is methylchloride;

8. The process claimed in claim 1 wherein said alkyl halide is ethylchloride.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Cellulose and Cellulose Derivaties, 2nd ed. (Ott et al.),Interscience Publishers, Inc., New York (1954), pp. 888, 895 and 937relied on.

1. THE PROCESS OF PREPARING ALKYL HYDROXYALKYL ETHERS OF CELLULOSE WHICHARE READILY SOLUBLE IN WATER, HAVE HIGHER GELATION TEMPERATURES THAN THEPRIOR KNOWN ETHERS HAVING THE SAME SUBSTITUENTS, AND THETEMPERATURE-VISCOSITY CURVES OF WHOSE SOLUTIONS ARE ESSENTIALLY FREEFROM HYSTERESIS ON HEATING AND COOLING, WHICH CONSISTS ESSENTIALLY INCAUSING CELLULOSE TO ABOSORB FROM ABOUT 1.2 TO ABOUT 2.8 MOLS OF AQUEOUSALKALI METAL HYDROXIDE PER MOL. OF CELLULOSE,AT A TEMPERATURE FROM ABOUT15* TO ABOUT 30*C., AND SUBJECTING THE RESULTING ALKALI CELLULOSE AT ATEMPERATURE FROM 20* TO 140*C. TO THE ETHERIFYING ACTION OF AN LKYLHALIDE HAVING FROM 1 TO 2 CARBON ATOMS, AN AMOUNT APPROXIMATELYEQUIVALENT TO THE OMOUNT OF SAID ABORBED ALKALI METAL HYDROXIDE, AND TOTHE ETHERIFYING ACTION OF AN ALKYLENE OXIDE HAVING FROM 2 TO 4 CARBONATOMS, UNTIL SUBSTANTIALLY ALL OF THE ALKYL HALIDE AND FROM ABOUT 0.1 TOABOUT 0.5 PART BY WEIGHT OF ALKYLENE OXIDE PER PART OF CELLULOSE HASREACTED, AND RECOVERING THE RESULTING MIXED ETHER OF CELLULOSE.